[TEP-0044] Update Alternatives

This commit describes design constraints related to running multiple Tasks in one pod in more detail.
It also updates the alternative solutions based on the clarified problem statement.

Changes:
- Add 3 options: pipeline in a pod + pipelines in pipelines, pipeline -> taskrun, and task pre and post steps
- Modify and add more detail to the "TaskGroup" proposal
- Remove "focus on workspaces" option: this adds new syntax but not new functionality
- Remove "custom pipeline" solution due to similarity with grouping CRD solution
- Remove "custom scheduler" option: this is a solution to a different problem

Co-authored-by: Christie Wilson christiewilson@google.com